1. Field of the Invention
The invention relates to a tunable resonance amplifier comprising a non-regenerative DC negative feedback loop, which incorporates first and second first-order low-pass RC filters, has a signal path arranged from an input to an output and a feedback path arranged from the output to the input and has a controlled loop gain.
The invention also relates to a frequency synthesis circuit comprising a frequency divider circuit, a phase detector, a loop filter and a voltage-controlled oscillator successively incorporated in a phase-locked loop.
The invention also relates to a superheterodyne receiver with an adaptive IF signal processing.
2. Description of the Related Art
A tunable resonance amplifier of this type is known, for example, from German Patent Specification No. 1,262,466.
A frequency synthesis circuit of this type is known, for example by the type indication LM 7005 of Sanyo.
A superheterodyne receiver of this type is known, for example, from U.S. Pat. No. 3,541,451.
In the signal path from the input to the output the known resonance amplifier successively comprises a voltage signal adding circuit, an inverting controllable voltage amplifier and an RC network composed of a cascade arrangement of the first and the second first-order low-pass RC filters. The output of the resonance amplifier is connected to the adder circuit via the feedback path, resulting in a DC negative feedback loop with a controllable loop gain.
The known resonance amplifier has a bandpass characteristic around a tunable resonance frequency which can be varied by controlling the loop gain. At this resonance or tuning frequency the frequency-dependent transmission of the resonance amplifier is maximum. The passbandwidth of this resonance amplifier is determined by the time constants of the two RC filters and is independent of the loop gain. This provides the possibility of varying. the tuning frequency and hence the frequency location of the pass range at a constant size of the bandwidth.
However, the known resonance amplifier has a selective gain which increases with an increasing tuning frequency, and conversely. Although the increase of the selective gain can be reduced with an increasing tuning frequency by means of an extra integrator arranged in series with the signal path, such an integrator introduces unwanted effects such as, for example a very strong increase of the output signal of the amplifier at a tuning frequency decreasing to zero. The field of use of this known tunable resonance amplifier is comparatively small, also because of this frequency-dependent selective gain. Application in, for example, receiver tuning devices for a tunable channel RF selection of a desired RF reception signal is not possible.